During normal sinus rhythm (NSR), the heart beat is regulated by electrical signals produced by the sino-atrial (SA) node located in the right atrial wall. Each atrial depolarization signal produced by the SA node spreads across the atria, causing the depolarization and contraction of the atria, and arrives at the atrioventricular (A-V) node. The A-V node responds by propagating a ventricular depolarization signal through the bundle of His of the ventricular septum and thereafter to the bundle branches and the Purkinje muscle fibers of the right and left ventricles.
Conduction defects may occur along the intrinsic conduction pathways of the heart leading to irregularities in heart rate and asynchrony between heart chambers. Cardiac pacemakers are available to deliver electrical pacing pulses to one or more heart chambers to restore a more normal heart rhythm. Cardiac pacemakers may be coupled to one or more medical electrical leads to position electrodes at desired pacing sites, e.g., at endocardial pacing sites or within a cardiac vein. Single chamber leadless pacemakers have been proposed that carry electrodes on the housing of the pacemaker and may be implanted in a heart chamber without requiring a transvenous lead. The single chamber leadless pacemaker may sense cardiac electrical signals that indicate depolarization of the heart chamber in which the pacemaker is implanted and deliver pacing pulses in the same cardiac chamber when intrinsic cardiac events such as R-waves are not being sensed by the pacemaker to provide bradycardia pacing.